Generally, broadcast transmitters such as those used in broadcasting radio frequency signals, when initially turned on (commonly referred to as "key-up"), behave differently compared to when the transmitter has been transmitting for a significant period of time. The primary reason for the variation in behavior is the change in temperature in the output transistors of the transmitter's power amplifier. Immediately following key-up, the power amplifier's output transistors typically are at room temperature. In contrast, with the transmitter transmitting after key-up, the output transistors heat up and reach a steady-state junction temperature of up to 200.degree. C. or more, when the transistors have appropriate heat dissipation mechanisms (e.g., heat sinks and/or fans). During this transistor "warm-up" period, the changing junction temperature of the output transistors generally affects the behavior of the power amplifier. However, the power amplifier's behavior is substantially predictable once the steady-state junction temperature is reached. Typically, the power amplifier's output transistors require about five to six hundred milliseconds to warm up to steady state junction temperature after key-up.
The transistor warm-up period following key-up can be problematic in that the different behavior during key-up can cause distortion of the transmitter output signal if the transmitter transmits during this period (referred to herein as the key-up period). In particular, for transmitters using predistortion linearization schemes, the key-up period behavior tends to be difficult for the predistortion algorithm to match. Consequently, during the key-up period, the linear transmitter's output signal is distorted. In addition, if the transmitter uses a training scheme to update the predistortion, the transmitter may undesirably train on abnormally distorted output signals during the key-up period. Conventional transmitters with "backed-off" power amplifiers may avoid key-up distortion, but at the cost of lower efficiency.
Moreover, in paging system applications, the distortions may cause the output signal to undesirably exceed Federal Communications Commission (FCC) adjacent channel emission limits (i.e., the output signal may have components with frequencies that are outside the assigned frequency band). To avoid transmitting distorted data that may cause reception errors, conventional paging system transmitters may transmit dummy data during the key-up period. However, the key-up period distortions may exceed the FCC limits and degrade the performance of these adjacent channels. Other conventional transmitters may transmit an unmodulated carrier signal during the key-up period, which does not produce adjacent channel emissions. In either of these implementations, no useful data is being transmitted during the key-up period, thereby undesirably reducing the transmitter's throughput. Accordingly, there is a need for a practical transmitter system that reduces both the key-up period distortion and the key-up time period.